VI.
PHYSICAL ACOUSTICS
Academic and Research Staff
Prof. K. U. Ingard
Dr. C. Krischer
Graduate Students
P. A. Montgomery
R. M. Spitzberg
A. G. Galaitsis
G. F. Mazenko
A.
OBSERVATION OF INTERACTION BETWEEN ACOUSTIC
SURFACE WAVES AND ELECTRON SURFACE WAVEST
There have been numerous studies of the amplification of acoustic surface waves
The process is
on a piezoelectric by drifting electrons in an adjacent semiconductor.
a nonresonant coupling of the acoustic wave to the electrons, and amplification occurs
over a broad frequency range.
In the presence of sufficiently large transverse magnetic fields, electron density
3
waves can propagate on a semiconductor surface.2 In a recent paper, Bers and Burke
showed theoretically that these electron surface waves can resonantly amplify the
acoustic surface waves: The predicted amplifications are considerably larger than
for the nonresonant case.
We have observed this resonant interaction and our results are in good agreement
with the theoretical predictions. Room-temperature measurements were made on an
LiNbO3-GaAs surface-wave amplifier at 30 MHz and 150 MHz, using magnetic fields
of up to 143 kG.
A paper describing these experiments
in detail is
being
prepared
for publi-
cation.
C. Krischer, A. Bers
References
1.
Yu. V. Gulyaev and V. I. Pustovoit, Soviet Phys. - JETP 20, 1508 (1965);
K. A. Ingebritsen, ELAB Report TE 94, Norwegian Institute of Technology,
Trondheim, Norway, 1967; K. Yoshida and M. Yamanishi, Japan J. Appl. Phys. 7,
1143 (1968); J. J. Collins, K. M. Lakin, C. F. Quate, and H. J. Shaw, Appl. Phys.
Letters 13, 314 (1968)
This work was supported principally by the U. S. Navy (Office of Naval Research)
under Contract N00014-67-A-0204-0019.
tThis work was supported in part by the National Science Foundation (Grant
GK-18185). Part of this work was performed while one of the authors (C. K.) was a Guest
Scientist at the Francis Bitter National Magnet Laboratory, M. I. T. , which is supported
by the Air Force Office of Scientific Research.
QPR No. 99
PHYSICAL ACOUSTICS)
(VI.
2.
R. Hirota, J.
Phys. Soc. Japan 23,
798 (1967); G.
312 (1968).
3.
A.
Bers and B. E.
B.
NONLINEAR
S Kino, Appl. Phys. Letters 12,
Burke, Appl. Phys. Letters 16, 300 (1970);
17,
47 (1970).
INTERACTION BETWEEN A SOUND FIELD
AND A LIQUID SURFACE
In an earlier progress report
we described a study of the nonlinear interaction
between a sound wave and a liquid surface and proposed a mechanism to explain
1Cl
(b)
Fig. VI-1.
QPR No. 99
(a) Nonlinear interaction of sound with a liquid in a
partially filled tube. Note that the liquid is pulled
up toward a partition that is inserted into the tube.
(b) An asymmetrical partition induces "rectification"
in the sound field, yielding different mean pressures on the two sides of the partition as indicated
by the difference in liquid level.
(VI.
PHYSICAL ACOUSTICS)
the rupture of the liquid surface which occurs at sufficiently high sound levels.
This
mechanism is based on the observation that the mean pressure in a sound field generally is smaller in the vicinity of a surface irregularity because
of the increase in
the local particle velocity caused by the diffracted near field.
To test this hypoth-
esis, we have carried out an experiment in which a transverse partition was introduced in
a duct,
as shown.
as shown in Fig.
VI-la.
The duct is partially filled with a liquid,
As the sound field is turned on, it is found that the liquid is sucked up toward
the partition, in support of the hypothesis.
In this context another experiment was tried in which a symmetrical partition was
The partition was placed close to the rigid termination
used, as shown in Fig. VI-ib.
of the duct so that a cavity resonator was formed. The opening formed between the partition and the liquid-covered floor of the duct has a different flow coefficient in the forward and backward directions.
Consequently,
in the nonlinear regime we expect that
an incident sound wave will give rise to mean pressure which will be different on the
two sides of the partition.
As a result of this asymmetry, an increase in the mean pres-
sure and a corresponding depression of the liquid surface are expected inside the cavity
when the partition is oriented in the manner shown in Fig. VI-1b.
was turned
on,
however,
we
decreased inside the cavity.
found that
the liquid level was
As the sound field
increased
rather
than
This paradoxical result is due presumably to vortex for-
mation by the sound field in the vicinity of the orifice, and, at present, we are exploring
this matter further.
U. Ingard
References
1. U. Ingard and J. A. Ross, Jr. , Quarterly Progress Report No. 95, Research Laboratory of Electronics, M. I. T. , October 15, 1969, pp. 30-37.
C.
TECHNIQUES
FOR MEASURING DENSITY FLUCTUATIONS
IN JETS
The use of gas-discharge
techniques for studying the shock fronts in supersonic
gas flows appears very promising.
been tried:
DC-
corona discharge
and RF-excited
probe.
Three methods of measuring the density have
gas discharges with Langmuir probes, and with a
The first two methods raised technical problems that made
their use undesirable, whereas the corona probe circumvents these limitations.
The
DC
discharge
method is
limited because
of the
large
fluctuations
of the
floating potential of the Langmuir probe, which make it difficult to attach the probe
to the recording equipment that must also be floated at the plasma potential.
The
RF discharge method is complicated by recombination of the electrons and ions in
QPR No. 99
(VI.
PHYSICAL ACOUSTICS)
the flow.
tion.
This masks the true density function of the gas under the ion decay func-
Moreover, RF excitation leads to severe noise problems.
The corona probe makes use of the corona discharge
at the point of a 5-mil
tungsten wire raised to a potential of 750 V above the ambient.
The discharge cur-
rent is a function of the pressure, which increases monotonically to a
maximum,
decreases, and then finally drops discontinuously to a small value.
The behavior
of the probe's characteristic curve warrants further study.
In the pressure region
of interest the curve is
single-valued, and, therefore, when calibrated, makes an
excellent pressure probe that has none of the limitations of either the DC or RF
discharge techniques.
R.
QPR No. 99
H. Price, U.
Ingard